Extend work items USAGE calculation to indirect work items

[pellecchialuigi]

Actually USAGE feature show only a list of software components where the work item has a direct relationship with.
To really understand where a work items is USED we should scan also indirect relationships, considering multiple level of Software Requirements also.
The USAGE feature is aimed to clarify all the software components that will be affected by a change of the work item.
This feature should be used in pair with the Fork one. So if someone need to apply a change to a work item only to a software component, the idea is to verify where the work item is used and:

• if only used in the current software component it will be safe to edit the work item
• if the work item is used across multiple software components and we want to apply a change only on the current one, we should fork it. So it inherits the history of the current work item but will become a new one, with a new id.